Modelling of electron beam scattering in high resolution lithography for the fabrication of Xray masks

A single-layer resist process for X-ray master mask fabrication by electron beam lithography is theoretically and experimentally investigated. In the mask fabrication process boron nitride membranes are utilized and final absorber structures are obtained by Au electroplating after e-beam patterning at different primary beam energies, on a single PMMA layer of 10000 Å. The e-beam energies experimentally utilized are 20 and 30 keV. Detailed Monte Carlo analysis of the multilayer structure is carried out at 10, 20, 30 and 40 keV, the corresponding proximity function calculated and compared to experiment. It is found that an important source of back-scattering is constituted by the thin metal layer as plating base, while the ultimate limit for resolution seems to be determined by forward scattering. Finally, it is demonstrated that by proper selection of the e-beam energy it is possible to obtain high contrast absorbers down to 0.3 µm lines and spaces.